There is a conventionally carried out medical practice in which a catheter is percutaneously inserted into a blood vessel and is introduced to an organ such as the brain, the heart, and the abdomen, thereby administrating and injecting, for example, a therapeutic agent, an embolic material, or a contrast medium, delivering, for example, an endoscope, another catheter, or a guide wire, or aspirating, for example, a blood clot. With the recent progress in medicine, for example, the treatment in a thinner peripheral blood vessel and a minimally invasive treatment using a catheter having a smaller outer diameter have been being carried out, and such a treatment requires a catheter having a smaller diameter but having higher performance than ever before. Examples of the performance of a catheter include pushability of reliably transferring a pushing force from an operator to the leading end of a catheter and reachability to a thin and bent peripheral blood vessel. For the injection of a medicinal agent, the aspiration of a blood clot, and the delivery performance of an endoscope or another catheter, the inner diameter of a catheter is very important. In addition, in order to carry out a minimally invasive treatment, the outer diameter is also required to be reduced, and hence an extremely thin-walled tube is required. Until now, it has been very difficult to ensure that such a thin-walled tube has kink resistance of preventing a catheter from being bent even at a bent part or a curved part of a blood vessel, inner cavity retention performance when a catheter is highly bent, and tensile strength for preventing a catheter from being easily broken when, for example, the catheter is caught in the body.
Conventionally, as a tube having excellent kink resistance and excellent inner cavity retention performance when the tube is highly bent, a resin tube including a coil structure as a reinforcement layer has been studied. The coil structure is extremely excellent in the kink resistance and the inner cavity retention performance when a tube is highly bent but is inferior in the tensile strength. Thus, in order to ensure the tensile strength of a tube using the coil structure, a resin tube is required to have a much heavier wall thickness or to include a resin having high rigidity. However, a heavy-walled tube has an increased outer diameter or a reduced inner diameter, and this raises problems in which, for example, such a tube cannot be inserted into a thinner peripheral blood vessel, cannot achieve a minimally invasive treatment, and causes the reduction of the injection performance, the aspiration performance, and the delivery performance. A tube including a resin having high rigidity generally has low toughness, and this raises problems in which the resin layer cracks when the tube is highly bent, thereby reducing the kink resistance and the tensile strength. Hence, such a tube cannot be safely used.
As a method for improving the tensile strength of a tube including a coil structure as a reinforcement layer, a method using an axial member in the longitudinal direction of a catheter is disclosed (Patent Document 1). Patent Document 1 describes an intravascular catheter that further includes an axial member extending along a reinforcement layer including a braid. It is described that the combination of the axial member can prevent a shaft from being elongated. In the structure, the axial member is not fixed to any polymer layer adjacent to the braid. Though the method surely suppresses the elongation in the axial direction, the strength of a wire of the axial member is required to be increased against a higher tensile force, and a catheter obtained by the method may cause anisotropic flexural rigidity. In addition, an embodiment in Patent Document 1 describes that, in the manufacturing process, a composite subassembly including the axial member, the braid, the polymer layer, and others is heated, thereby fusing and compressing the members to each other.
Another disclosed method is a method of providing a braided structure on the outside of a coil structure (Patent Document 2). In Patent Document 2, a flat square braided part made of metal is provided on the outside of a flat plate-shaped tightly wound coil made of metal, and a resin covering layer is further provided on the outside of the flat square braided part. The tube intends to satisfy both the compressive force resistance due to the coil structure when the tube is bent and the tensile force resistance due to the braided structure. However, when the thickness or the width of a wire constituting the braid is increased against high tensile force, the compressive force resistance when the tube is bent, obtained by the coil structure is reduced. Therefore, such a tube is difficult to be applied to the leading end of a catheter that is required to have flexibility and high tensile force.
These techniques have structures requiring a reinforcement layer in addition to the coil layer and the resin layer and hence are absolutely impossible to be applied to a thin-walled tube.